Threading-dislocation-free nanoheteroepitaxy of Ge on Si using self-directed touch-down of Ge through a thin SiO2 layer

ABSTRACT

A method of forming a virtually defect free lattice mismatched nanoheteroepitaxial layer is disclosed. The method includes forming an interface layer on a portion of a substrate. A plurality of seed pads are then formed by self-directed touchdown by exposing the interface layer to a material comprising a semiconductor material. The plurality of seed pads, having an average width of about 1 nm to 10 nm, are interspersed within the interface layer and contact the substrate. An epitaxial layer is then formed by lateral growth of the seed pads over the interface layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Patent ApplicationSer. No. 60/501,274 filed on Sep. 9, 2003, the disclosure of which isincorporated herein by reference.

GOVERNMENT RIGHTS

This invention was made with government support under Award No.DMR-0094145 awarded by the National Science Foundation. The governmenthas certain rights in the invention.

FIELD OF THE INVENTION

The present invention relates to. semiconductor devices and methods fortheir manufacture and, more particularly, relates to epitaxial growth oflattice mismatched systems.

BACKGROUND OF THE INVENTION

Conventional semiconductor device fabrication is generally based ongrowth of lattice-matched layers. A lattice mismatched epitaxial layerat a semiconductor interface can lead to a high density of dislocationsthat degrade semiconductor device performance. Over the past severalyears, however, there has been increased interest in epitaxial growth oflattice-mismatched semiconducting material systems. Lattice mismatchedsystems can provide a greater range of materials characteristics thansilicon. For example, the mechanical stress in a lattice mismatchedlayer and control of its crystal symmetry can be used to modify theenergy-band structure to optimize performance of optoelectronic devices.Lattice mismatched systems can also enable compound semiconductordevices to be integrated directly with Si-based complementary metaloxide semiconductor (CMOS) devices. This capability to formmultifunction chips will be important to the development of futureoptical and electronic devices.

Problems arise, however, because an epitaxial layer of alattice-mismatched material on a substrate is often limited to acritical thickness (h_(c)), before misfit dislocations begin to form inthe epitaxial material. For example, h_(c)=2 nm for a germaniumepitaxial layer on a silicon substrate. Because of the relatively smallh_(c) and the large dislocation densities at thicknesses greater thanh_(c), use of the heteroepitaxial layer is impractical.

Thus, there is a need to overcome these and other problems of the priorart and to provide a method to grow defect free heteroepitaxial layersof lattice mismatched systems.

SUMMARY OF THE INVENTION

According to various embodiments, the present teachings include a methodof forming a semiconductor layer including forming an interface layer ona portion of a substrate. The interface layer is exposed to a materialcomprising a semiconductor material to form a plurality of seed padsinterspersed within the interface layer and contacting the substrate. Asemiconductor layer is then formed by lateral growth of the seed padsover the interface layer.

According to various embodiments, the present teachings also include amethod of forming a semiconductor layer including forming an oxide layeron a portion of a semiconductor substrate. A plurality of seed padscomprising germanium are formed by self-directed touchdown. Theplurality of seed pads are interspersed within the oxide layer andcontact the semiconductor substrate. A germanium layer is then formed bylateral growth of the seed pads over the oxide layer.

According to various embodiments, the present teachings further includea semiconductor device including a substrate and an oxide layer disposedon a portion of the substrate. The semiconductor device also includes aplurality of seed pads formed by self-directed touchdown. The pluralityof seed pads are separated from each other by the oxide layer. Thesemiconductor device further includes a semiconductor layer disposedover the substrate, wherein the semiconductor layer is formed by lateralgrowth of the seed pads over the oxide layer.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory onlyand are not restrictive of the invention, as claimed.

The accompanying drawings, which are incorporated in and constitute apart of this specification, illustrate several embodiments of theinvention and together with the description, serve to explain theprinciples of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a cross-section of a substrate in accordance withexemplary embodiments of the invention.

FIG. 2 depicts a cross-sectional view of an interface layer on asubstrate in accordance with exemplary embodiments of the invention.

FIG. 3A depicts a cross-sectional view of seed pad sites interspersed inan interface layer in accordance with exemplary embodiments of theinvention.

FIG. 3B depicts a top-down view of seed pad sites interspersed in aninterface layer in accordance with exemplary embodiments of theinvention.

FIG. 4A depicts a cross-sectional view of seed pads on a substrate andseparated by portions of the interface layer in accordance withexemplary embodiments of the invention.

FIG. 4B depicts a top-down view of seed pads on a substrate andseparated by portions of the interface layer in accordance withexemplary embodiments of the invention.

FIG. 5 depicts a cross-sectional view of a semiconductor layer formed bylateral growth of seed pads over portions of the interface layer inaccordance with exemplary embodiments of the invention.

FIG. 6 depicts a cross-sectional view of a second semiconductor layer onthe semiconductor layer in accordance with exemplary embodiments of theinvention.

DESCRIPTION OF THE EMBODIMENTS

In the following description, reference is made to the accompanyingdrawings that form a part thereof, and in which is shown by way ofillustration specific exemplary embodiments in which the invention maybe practiced. These embodiments are described in sufficient detail toenable those skilled in the art to practice the invention and it is tobe understood that other embodiments may be utilized and that changesmay be made without departing from the scope of the invention. Thefollowing description is, therefore, not to be taken in a limited sense.

As used herein, the term “self-directed touchdown” refers to anucleation and growth process that is initiated without reliance on aphotolithographic mask to pattern a substrate or other layer.

As used herein, the term “nanoheteroepitaxy” refers to engineering aheterojunction at the nanoscale to relieve lattice strain.

FIGS. 1 to 6 depict exemplary semiconductor devices withnanoheteroepitaxial layers and manufacturing methods to formsemiconductor devices with nanoheteroepitaxial layers having a latticemismatched substrate and semiconductor layer in accordance with variousembodiments of the invention. The semiconductor devices comprise aplurality of seed pads formed by self-directed touch-down through aportion of an interface layer and on a substrate. A semiconductor layercan then be formed by coalescence of the seed pads over the portions ofthe interface layer. The semiconductor layers formed by the exemplarymethods have a sufficiently low threading-dislocation density,regardless of the critical thickness of the semiconductor layer, so thatthe deposited semiconductor layer can be effectively used for integratedcircuit applications. For ease of illustration, the invention will bedescribed with reference to a manufacturing process for formation of anepitaxial layer of germanium (Ge), having a lattice parameter of about5.66 Å, on silicon (Si), having a lattice parameter of about 5.44 Å.

Referring to FIG. 1, a substrate 10 is shown. Substrate 10 can be, forexample, a silicon substrate. Other substrate materials can include anysemiconductor material having a lattice-mismatch to a desired epitaxiallayer. The terms “lattice-mismatch” and “lattice-mismatched material” asused herein refer to any materials whose lattice parameters in a givencrystalline plane or direction are not identical. Lattice-mismatchedmaterials can include, but are not limited to, silicon and germanium,silicon and carbon, silicon and GaAs, silicon and InP, and silicon andgallium nitride.

As shown in FIG. 2, an interface layer 20 can be formed on substrate 10.Interface layer 20 can be, for example, an oxide layer, such as, a SiO₂layer, having a thickness of about 1 Å to about 30 Å. The SiO₂ layer canbe formed by methods known in the art, such as, treating substrate 10 ina Piranha solution or by thermal growth. Other interface materials caninclude, but are not limited to, silicon nitride, siliconoxynitride,anodized aluminum oxide (AAO), and other oxides. In various embodiments,interface layer 20 can comprise an amorphous material.

In various embodiments, properties of interface layer 20, such assurface roughness and thickness, can be controlled to tailor the defectmorphology of the epitaxial layer. For example, interface layer 20 canbe formed using H₂O₂ to achieve a monolayer of atomically flat SiO₂ on ahydrogenated Si(100) substrate.

After forming interface layer 20 on substrate 10, interface layer 20 canbe exposed to a material comprising a semiconductor material. Exposuretemperatures can be about 500° C. to about 750° C. The semiconductormaterial can comprise, for example, Ge. In various embodiments,molecular beam epitaxy can be used to expose interface layer 20 to Ge.As shown in FIGS. 3A and 3B, the Ge can react with interface layer 20,to form interface layer free areas 30, exposing portions of substrate10. Interface layer free areas 30 can form, for example, throughreaction of the Ge with the oxide film as follows:SiO2(s)+Ge(ad)→SiO(g)+GeO(g). Interface layer free areas 30 can berandomly distributed to form a remaining portion of interface layer 25,and can be about 2 nm to about 8 nm wide. The spacing between interfacelayer free areas can be about 2 nm to about 14 nm.

As exposure to Ge by molecular beam epitaxy continues, Ge can deposit ininterface layer free areas 30. There is generally no deposition onremaining portions of interface layer 25, due to selective deposition.This self-directed touch-down of Ge on Si occurs without lithography topattern the substrate or interface layer. The regions of Ge growth on Sisubstrate 10 can form crystalline Ge islands, referred to herein as seedpads 40, shown in FIGS. 4A and 4B. Seed pads 40 can then laterallyovergrow and coalesce, as exposure to Ge continues. As lateral growth ofGe seed pads 40 over remaining SiO₂ oxide layer 25 continues, Ge seedpads 40 coalesce into a single semiconductor layer. The semiconductorlayer can be a virtually defect free single crystalline epitaxiallateral overgrowth (ELO) layer.

FIG. 5 shows a semiconductor layer 50 formed by coalesced seed padshaving an atomically abrupt interface with substrate 10. Semiconductorlayer 50 can be virtually defect-free having a threading dislocationdensity of about 1×10⁵ cm⁻² or less. Stacking faults can exist over theremaining oxide layer patches 25, but generally terminate within about80 nm from the SiO₂-Ge interface. The thickness of semiconductor layer50 can be greater than the critical thickness h_(c), for example,greater than the critical thickness of 2 nm for 100% Ge on Si. Seed pads40 can have an average width of about 1 nm to 10 nm. The distancebetween seed pads can be about 3 nm or more.

In various embodiments, a second semiconductor layer 60 can be depositedover the semiconductor layer. The second semiconductor layer cancomprise one or more materials from Group II, Group III, Group IV, GroupV, and Group VI, such as, for example, GaN, GaAs, AlGaAs, InGaP, AlInP,AlInGaP, InGaAsN, SiGe. As shown in FIG. 6, second semiconductor layer60 can be formed on semiconductor layer 50.

A specific example of nanoheteroepitaxy to grow virtuallydislocation-free lattice mismatched materials will now be provided. Itis to be understood that the disclosed examples are exemplary and in noway are intended to limit the scope of the invention.

EXAMPLE 1

Samples I was undoped Si(100) including an interface layer of SiO₂.Sample II was undoped Si(100) stripped of an interface layer by exposureto HF.

Both sample substrates were first treated to remove contaminates byimmersion in a Piranha solution for about 5 minutes to form a thin layerof SiO₂ on the substrate. The Piranha solution was prepared by mixing 4volumetric parts of 2M H₂SO₄ with 1 volumetric part of 30 wt % H₂O₂. Thesamples were then treated for 5 minutes in an HF solution to remove theSiO₂ layer. The HF solution was prepared by diluting a 49 wt %electronics grade HF solution to 11 wt % by deionized H₂O. ThePiranha-HF treatments were repeated three times.

A fresh Piranha solution was prepared and Sample I was treated at 80°for 10 minutes to form a thin layer of chemical oxide. After treatment,Sample I was rinsed with deionized H₂O, and blow-dried with N₂. Nochemical oxide layer was formed on Sample II.

Sample I was then immediately loaded into an ultrahigh vacuum molecularbeam epitaxy (UHV-MBE) chamber having a base pressure of 4×10⁻¹⁰ Torr.Sample II was immediately loaded into the UHV-MBE chamber after thethird HF treatment that removed the oxide layer. Both of the sampleswere heated to 620° C. and a Knudsen effusion cell was used to exposethe samples to Ge. The effusion cell was operated at 1200° C. for a Gegrowth rate of 100 nm/hour. During growth of Ge, the chamber pressurewas below 1×10⁻⁹ Torr.

Cross sectional transmission electron microscopy (XTEM) showed that, inSample I, amorphous oxide regions were interspersed with Ge seed pads. Asingle crystalline Ge ELO layer, approximately 4 μm thick, covered theamorphous oxide regions and seed pads. The thickness of the oxide layerregions in Sample I was determined to be 1.2 nm. The average dimensionof the seed pads was about 7 nm, and the spacing between the seed padsranged from 4 nm to 12 nm.

XTEM further showed that the Ge layer of Sample II, having no seed pads,contained a network of dislocation segments primarily within 250 nm fromthe Ge-Si interface. In contrast, the Ge ELO layer of Sample I was freeof a dislocation network. Only stacking faults emanating from the seedpad-Ge interface existed along the {111} planes in the Ge ELO layer ofSample I. Those stacking faults terminated within 80 nm of theinterface. As a result, Sample I, fabricated with the oxide layer thatresulted in seed pads, formed an atomically abrupt Ge-Si interface and avirtually defect free Ge ELO layer. Sample I had a threading dislocationdensity of about 1×10⁵ cm⁻² or less. Sample II, however, formed withoutthe oxide layer and seed pads, formed a Ge layer containing a network ofdislocation segments.

Other embodiments of the invention will be apparent to those skilled inthe art from consideration of the specification and practice of theinvention disclosed herein. It is intended that the specification andexamples be considered as exemplary only, with a true scope and spiritof the invention being indicated by the following claims.

1. A method of forming a semiconductor layer comprising: forming anoxide layer on a portion of a semiconductor substrate; exposing theoxide layer to a material comprising germanium such that a plurality ofwindows are formed in the oxide layer exposing portions of thesemiconductor substrate and such that a plurality of seed padscomprising germanium are formed by self directed touchdown, wherein theplurality of seed pads are formed in the plurality of windows andcontact the semiconductor substrate; and forming a germanium layer bylateral growth and coalescence of the seed pads such that the germaniumlayer is disposed over the oxide layer, wherein each of the plurality ofwindows in the oxide layer is about 2 nm to about 8 nm wide, and whereinthe germanium layer has a threading dislocation density of less than1×10⁵ cm⁻².
 2. The method of forming a semiconductor layer of claim 1,wherein the plurality of seed pads are separated from each other byabout 1 nm to 15 nm.
 3. The method of forming a semiconductor layer ofclaim 1, wherein an average seed pad width is about 1 nm to 10 nm. 4.The method of forming a semiconductor layer of claim 1, wherein theoxide layer is about 1 Å to 30 Å thick.
 5. The method of forming asemiconductor layer of claim 1, wherein the semiconductor substratecomprises silicon.
 6. The method of forming a semiconductor layer ofclaim 1, further comprising forming a semiconductor layer over thegermanium layer.
 7. The method of forming a semiconductor layer of claim1, wherein the step of forming a germanium layer by lateral growth ofthe seed pads over the oxide layer forms a germanium layer having athickness greater than 2 nm.